The invention relates to a method and an arrangement for determining the rotation speed of a motor fed by an inverter.
An inverter is an electrical device which enables generation of voltage with changeable frequency. Typically, inverters are used in connection with motors for controlling them at a changing frequency. An inverter may also be a part of a frequency converter used for controlling a motor, for example.
In connection with inverters and, in general, motor drives comprising inverters, various safety functions are generally applied, the purpose of which is to monitor and ensure the safety of the motor drives in different operating situations and conditions. Examples of safety functions according to recommendations include Safe Torque Off STO, Safe Stop 1 SS1 and Safety Limited Speed SLS.
Implementing safety functions requires that the rotation speed of the motor be determined as reliably as possible. Typically, the rotation speed information on the motor is primarily obtained from the inverter controlling the motor. However, the rotation speed information obtained from the inverter alone does not necessarily guarantee the reliability of the safety functions, for example in a case where the inverter has failed in some way. Due to this, there is a need to arrange alternative redundant speed determination independent of the above-mentioned speed determination to implement the safety functions reliably.
One way to determine the rotation speed of the motor independently of the inverter feeding it is to measure the rotation speed from the motor by means of a tachometer, i.e. a rev counter. A tachometer may be connected to a motor, for example to its shaft, either with a physical connection or by means of an optical connection, for example, without physical contact, thus determining the rotation speed of the motor.
A problem relating to the determination of the rotation speed of a motor is that implementing the safety functions always requires an additional tachometer for each motor to be monitored, which increases the costs. Further, connecting an additional tachometer to the motor and/or positioning it in connection with a motor, as well as maintenance possibly required for the tachometer positioned in connection with the motor may be difficult in some conditions.